1. Field of the Invention
This invention relates to a capacitive-type sensor apparatus for detecting a physical quantity.
2. Description of the Related Art
A typical capacitive sensor apparatus includes a capacitor designed so that its capacitance will depend on a physical quantity to be detected. The typical capacitive sensor apparatus further includes a circuit for detecting the capacitance of the capacitor. The detected capacitance indicates the physical quantity.
Some capacitive sensor apparatuses are of integrated structures. In such an apparatus, when a capacitor therein fails, it is difficult to replace it with new one.
It is an object of this invention to provide a fault-tolerant capacitive sensor apparatus, that is, a capacitive sensor apparatus which can implement failure compensation.
A first aspect of this invention provides a capacitive sensor apparatus comprising a capacitive sensor including a plurality of physical-quantity-detection capacitors each having a movable electrode and a fixed electrode; a conversion device for converting an output signal of the capacitive sensor into an apparatus output signal; first means for selectively connecting and disconnecting each of the physical-quantity-detection capacitors to and from the conversion device; second means for determining whether or not each of the physical-quantity-detection capacitors fails in response to the sensor output signal; and third means for, when the second means determines that first one of the physical-quantity-detection capacitors fails, disconnecting the first one from the conversion device and connecting second one of the physical-quantity-detection capacitors to the conversion device.
A second aspect of this invention is based on the first aspect thereof, and provides a capacitive sensor apparatus further comprising a diagnosis device for determining whether the apparatus output signal is normal or abnormal, and means for, when the diagnosis device determines that the apparatus output signal is abnormal, disconnecting currently-connected one of the physical-quantity-detection capacitors from the conversion device and connecting another of the physical-quantity-detection capacitors to the conversion device.
A third aspect of this invention is based on the first aspect thereof, and provides a capacitive sensor apparatus wherein the conversion device includes means for changing a characteristic of the conversion of the output signal of the capacitive sensor into the apparatus output signal in accordance with which of the physical-quantity-detection capacitors is connected to the conversion device to make the apparatus output signal independent of which of the physical-quantity-detection capacitors is connected to the conversion device.
A fourth aspect of this invention is based on the third aspect thereof, and provides a capacitive sensor apparatus wherein the means in the conversion device changes a gain of the conversion of the output signal of the capacitive sensor into the apparatus output signal in accordance with which of the physical-quantity-detection capacitors is connected to the conversion device.
A fifth aspect of this invention is based on the first aspect thereof, and provides a capacitive sensor apparatus further comprising means for applying a detection-purpose voltage to the capacitive sensor, and means for changing the applied detection-purpose voltage in accordance with which of the physical-quantity-detection capacitors is connected to the conversion device to make the apparatus output signal independent of which of the physical-quantity-detection capacitors is connected to the conversion device.
A sixth aspect of this invention provides a capacitive sensor apparatus comprising first and second capacitors having capacitances depending on a physical quantity to be detected; first means for detecting the physical quantity in response to the capacitance of the first capacitor; second means for determining whether or not the first capacitor fails; and third means for, in cases where the second means determines that the first capacitor fails, detecting the physical quantity in response to the capacitance of the second capacitor.